Why do some SEO companies offer very low prices?

A "low price" depends on what you consider to be a low price; this is very subjective. However, I will try to shed some light on the subject.
If by low, you mean something like "$50 per month" or "$500 per year" it's because they are either doing no work and charging you a premium, or they are using methods that will hurt your business. Low prices sound great, but if you factor in the costs of doing business, the hourly rate, etc. it is impossible for any company to do an effective job for "$50 per month."
For the average client, it takes about 1-4 hours per week for proper search engine optimization. This equals to 4-16 hours per month on average. So if they are charging you $50 per month, I promise they are not doing anything, or they don't know what they are doing. So, are their rates really that low, or are they simply a "supply and demand" business; you want the service, they are offering it. That doesn't mean they will perform the service, and since it's difficult to track what is being done, you might never discover you are being ripped off. Do a search for SEO and find out what the search engines have to say on this issue so you can avoid a costly mistake. I am not trying to put down SEO providers, but in my experience, 90% or more do not know what they are doing, and don't really do the work they say they are. As a consultant, I have found this to be true in every case.

That is why we offer Search Engine Optimization at REAL reasonable rates and do honest work. We will sit down and show you the results. We have years experience with real results so when you hire us for SEO, you will know what you are paying for, and will discover the benefits the longer you retain our services. Our goal is long term success, not short term cash flow. The better you do, the better we do. Our lowest plan is $125 per month, and includes 3 hours. Just for clarification, that's under $50 an hour. We spend about 15 minutes on  your website, 3 days per week. That is the minimal amount of time needed to effectively perform SEO. Our next plan up, which is $250 for 6 hours, is our best value, and we would recommend it for most of our customers.

Despite the fact that we have an additional 3 plans, even if we CAN charge you the money, we don't if it is unnecessary. For example, if your business is set on paying for our $500 per month plan, and we think you will only need our $250 plan, we will put you on our more inexpensive plan. We would we do that? Because our goal, is to establish long-term relationships. Not buy being cheap, nor by being overly expensive, selling you stuff you don't need. We want to be reasonable, honest, and fair. We know, as you work with us, you will recognize this in our business practices; as our other customers here in New Hampshire have noticed, and as our customers in the west did in the past before we moved to this great state.

So, we are charging real rates, and doing real work. What I can say is, in all cases, every single one, that a customer has trusted us, they have been happy they did.